(1) Field of the Invention
The invention relates to a method for carrying out a mass polymerization of monomers and/or prepolymers which are at least partly soluble in their polymers, in particular of methyl methacrylate (MMA), at least one monomer or prepolymer being metered into a reactor.
(2) Prior Art
The polymerization in a kneading reactor with or without solvent was also investigated and is described in DE 101 40 217.1/EP 10122571.7.
Polymerization is understood as meaning the conversion of low molecular weight compounds (monomers, oligomers) into high molecular weight compounds (polymers, macromolecules). Thus, the industrial production of polymers can be effected under heterogeneous or homogeneous conditions. Heterogeneous reactions are understood as meaning polymerization in suspension/emulsion, precipitation reactions and the fixed-bed procedure.
The suspension/emulsion polymerization methods are usually carried out in stirred vessels (viscosity is low). Large amounts of water and different additives are, however, required in order to produce segregation/heterogeneity.
The precipitation polymerization is frequently effected using thermodynamic separation of the polymer from its monomer and/or a solvent. A precondition for this is that the polymer is insoluble in the monomer or the solvent. In this method of production, the polymer is precipitated.
The continuous fixed-bed procedure operates with back-mixing. The already reacted polymer is mixed back in the direction of metering. This gives a heterogeneous mixture between liquid metering and prepared (polymerized) product. The result is a heterogeneous granular polymer material in which the liquid monomer or prepolymer completely polymerizes around the solid particles, these solid particles already having been polymerized.
Homogeneous mass polymerization is understood as meaning that the polymer forms a homogeneous solution in its monomer. The first condition to be fulfilled is that the polymer is readily soluble in its monomer. Such a defined homogeneous polymerization system can also operate with the above-mentioned fixed-bed procedure, with the result that the polymerization method takes place heterogeneously in this case because the monomer or prepolymer metered in has no time properly to dissolve its back-mixed granular polymer. Here, the limitation is the diffusion of the monomer into the polymer particles.
During the homogeneous mass polymerization of methyl methacrylate (MMA) with high monomer conversion, the polymer forms a solid or highly viscous polymer material which can block or shut down any conventionally operated reactor (e.g. CSTR, stirred vessel).
In order to overcome these mixing problems (limitations of heat transfer and mass transfer lead to local overheating and concentration segregation) which are associated with the highly viscous polymer material, various possible methods were proposed in the prior art for the homogeneous polymerization of MMA. Among these useful possibilities were:                a) the polymerization with high monomer conversion carried out at high temperature in order to reduce the melt viscosity. However, some parameters, such as the ceiling temperature (at which an equilibrium occurs between polymerization and depolymerization), can influence this possibility (Fleury P. A., Meyer T., Renken A., Dechema (1992), Volume 127, VCH, “Methyl methacrylate polymerization in a tubular reactor: product quality and reactor behavior”). Sulzer Chemtec has, for example, developed a continuous, homogeneous polymerization of styrene and MMA (methyl methacrylate) in a circulation reactor having static mixing elements. This method requires high temperatures (>>Tg, well above the glass transition temperature) in order to keep the viscosity of the reaction material low and hence to avoid deposits on the reactor walls and on the mixing elements. However, such high temperatures have undesired side effects, for example the formation of oligomers or depolymerization. In the case of styrene, these side effects are not problematic; however, for MMA, the depolymerization begins at as low as 155° C. (stabilizers are then required).        b) Dilution of the polymerization mixture with a large amount of an inert, liquid phase (solvent), which prevents the solidification of the reaction material for the polymerization with high monomer conversion and at low/medium temperature (close to Tg). The disadvantage of this method is the necessity of providing an expensive separation step with solvent recovery/treatment. Moreover, the use of solvents results in both a lower molecular weight and a reduced polymerization rate. For quality reasons, the PMMA producers do not wish the presence of solvents.        c) Polymerization in a CSTR (continuous stirred tank reactor) without a solvent at a medium temperature (close to Tg) up to a maximum monomer conversion of from 50 to 60%. The monomer conversion is limited in order to avoid a run away reaction in which the Trommsdorff effect causes stability problems in a conventionally stirred tank reactor. The large amounts of unreacted monomer must be separated from PMMA in an extruder and recycled. This is the conventional preparation method.        